1. Field of the Invention
The present invention relates generally to a PLL (phase-locked loop) frequency synthesized receiver and is directed more particularly to a PLL frequency synthesized receiver which can shorten an up/down broadcast station selection time.
2. Description of the Prior Art
There are presently known frequency-synthesized receivers which use a phase-locked loop (PLL) circuit as a local oscillator circuit. The PLL circuit normally includes a programmable frequency divider whose dividing ratio determines the local oscillation frequency. In such a receiver, a station selection code is provided to correspond to a desired received frequency. This code is stored or generated in a control apparatus, which can be a microcomputer, and is supplied to a data register coupled with the programmable divider. The contents of the data register then determine the dividing ratio of the programmable divider to determine the local oscillator frequency and, hence, to determine the received frequency.
If it is desired to change the tuning of the receiver by continuously sweeping the local oscillation frequency up or down, the contents of the data register must be changed by unit steps to correspond, for example, to frequency changes of 100 KHz.
Unfortunately, each up or down step requires a rather involved operation and consumes an inordinate amount of time. This prevents the receiver from changing frequencies rapidly.
For example, to change the frequency by one increment of 100 KHz (i.e., to change the dividing ratio of the programmable divider by "1") the code stored in the control apparatus, which code has been converted to a 16-bit BCD signal to control the contents of the data register, is converted back to binary form, and then is incremented or decremented. Then the incremented or decremented code is converted again to a BCD signal and is transferred to the data register. As the code can be handled only on a bit-by-bit basis, this operation consumes a great deal of time. In the case of a received frequency change from one end of the frequency band to the other, this operation can be annoyingly long.